Single-Input Single-Output Communication System
A traditional wireless communication system usually employs a single transmit antenna and a single receive antenna, i.e., a so-called single-input single-output (SISO) communication system. In the traditional single-input single-output communication system, radio-frequency (RF) modulated data from a transmitter located at abase station arrive at a receiver at a mobile station along a transmission path. However, characteristics of the transmission path often vary with time due to several factors like fading and multi-channel. Moreover, channel capacity of the traditional single-input single-output communication system is subjected to an insurmountable bottleneck, namely Shannon capacity restriction.
Multiple-Input Multiple-Output Communication System
To eliminate the above-discussed drawback in the traditional single-input single-output communication system, multiple-input multiple-output (MIMO) communication systems have been proposed and developed. FIG. 1 illustrates a schematic view of a MIMO communication system in the prior art. As illustrated in FIG. 1, the MIMO communication system usually comprises a transmitter with a number (NT) of transmit antennas, located at a base station, and a receiver with a number (NR) of receive antennas, located at a mobile terminal. MIMO channels formed of the NT number of transmit antennas and the NR number of receive antennas can be decomposed into an NS number of individual sub-channels, in which NS≦min {NT, NR}. Each of the NS number of individual sub-channels is further termed a control sub-channel of MIMO communication system and corresponds to one-dimensional space. With additional dimensions established by multiple transmit antennas and multiple receive antennas, the MIMO system can provide improved performance, such as increased transmission capacity. Further, in the MIMO communication system, individual data flows can be respectively transmitted on the NS control sub-channels, so that utilization efficiency of spectrum is increased.
Thus, the MIMO communication system, when compared with the traditional SISO communication system, can increase capacity of the communication system and utilization efficiency of spectrum by times without adding bandwidth. MIMO technique has become one of key techniques that will be used by the new and next-generation mobile communication systems. At present, several standards, such as WLAN (IEEE802.11n), WiMAX (IEEE802.16d and IEEE802.16e), IEEE802.20, IEEE802.22, 3GPP Release 7, and 3GPP Release 8 (LTE), have specified support for MIMO.
Due to the size, power, and cost limitations, in the MIMO communication system, the mobile terminal is configured with fewer antennas and RF channels than the base station, e.g., two transmit or receive antennas at the mobile terminal while four transmit or receive antennas at the base station. To make full use of the antennas at the base station, a variety of enhancement techniques are presented for the MIMO communication system. The existing enhancement techniques can be classified into two major categories: open-loop ones and closed-loop ones.
In the open-loop MIMO enhancement technique, the transmitter does not utilize MIMO channel information but directly transmits input data from itself to the receiver over MIMO channels. Typically, the open-loop enhancement technique comprises space-time coding (STC) technique and cyclic delay diversity (CDD) technique.
In the closed-loop MIMO enhancement technique, on the contrary, the transmitter has a prior knowledge of all or part of channel information and exploits the information to improve system performance. Specifically, the transmitter pre-processes input data by using channel information or user location information and transmits the pre-processed data to the receiver through a plurality of transmit antenna diversities. The receiver receives the transmitted data over receive antennas, processes and outputs the received data. Hence, the closed-loop MIMO enhancement technique outperforms the open-loop MIMO enhancement technique. Typically, the closed-loop MIMO enhancement technique comprises beamforming MIMO technique and pre-coding MIMO technique.
Combination of MIMO Communication System and Beamforming Technique
The beamforming MIMO technique generates beams with specific spatial orientation by using antennas, so as to enhance antenna gain and interference suppression gain. Usually, the beamforming MIMO technique is based on different locations of a target user and interference users and steers the main beam to the target user while aiming the side lobe or nulling beam at the interference users. In this manner, the beamforming technique can effectively reduce negative impact exerted by interference. FIG. 2 illustrates a block diagram of a MIMO communication system in the prior art comprising a beamformer. As illustrated in FIG. 2, a coder 11 performs channel coding, constellation modulation, and MIMO coding on input data in order to form one or multiple-channel coded data. Next, the coded data are input to a beamformer 12. At the same time, beamforming matrix generation means 13 generates a beamforming matrix for each transmit antenna and each channel of coded data according to information like direction of arrival (DOA) and provides the generated beamforming matrix to the beamformer 12. The beamformer 12 weights and sums the coded data by using the beamforming matrix, i.e., first weights each channel of coded data by a beamforming vector, and then sums the obtained weighted results of all channels of coded data for each transmit antenna as transmitted data corresponding to this transmit antenna. The coded data which have been weighted and summed are transmitted over transmit antennas. At the receiver, receive antennas receive the transmitted data. MIMO detection means 14 processes and outputs the received data. methods for generating beamforming weighted vectors and beamforming techniques have been disclosed in Chinese Patent Applications CN1653721, CN1689249, CN1864347, and CN1835416, which are hereby incorporated as reference.
The beamforming technique can provide interference suppression gain and is preferred in strong interference environments. However, in the case of weak interference, e.g., in a communication system with high frequency reuse factor, such beamforming technique benefits quite little.
Combination of MIMO Communication System and Pre-Coding Technique
In the pre-coding MIMO technique, the transmitter can configure MIMO transmission parameters in advance according to instantaneous or long-term MIMO channel information and through matrix coding, so that data are respectively transmitted over individual sub-channels, and diversity gain is achieved. Usually, the transmitter obtain MIMO channel information according to symmetry of uplink and downlink channels of the communication system or feedback channels. FIG. 3 illustrates a block diagram of a MIMO communication system in the prior art comprising a pre-coder. As illustrated in FIG. 3, a coder 21 performs channel coding, constellation modulation, and MIMO coding on input data in order to form one or multiple-channel coded data. Next, the coded data are input to a pre-coder 22. At the same time, pre-coding matrix generation means 23 generates a pre-coding matrix according to estimated channel matrix information for example and provides the generated pre-coding matrix to the pre-coder 22. The pre-coder 22 pre-codes the coded data by using the generated pre-coding matrix, i.e., first weights each channel of coded data by a pre-coding vector, and then sums the obtained weighted results of all channels of coded data for each transmit antenna as transmitted data corresponding to this transmit antenna. The pre-coded data are transmitted over individual sub-channels over transmit antennas. At the receiver, receive antennas receive the transmitted data. MIMO detection means 24 processes and outputs the received data. Methods for generating a pre-coding matrix according to matrix information have been disclosed in Chinese Patent Applications CN1756119A, CN1890908, CN1832369, CN 1838556, CN1941660, and CN1941661, which are hereby incorporated as reference.
It is seen pre-coding MIMO technique can reduce negative impact of channel fading or spatial correlation of channels on the MIMO communication system, thereby providing diversity gain. However, the pre-coding MIMO technique has no interference suppression gain capability, so it is rather deficient in strong interference environments, such as a MIMO communication system with a frequency reuse factor of 1.
In summary, although the existing beamforming MIMO technique has fine interference suppression capability, it cannot provide sufficient diversity gain. On the other hand, although the existing pre-coding MIMO technique provides fine diversity gain to eliminate negative impact of channel fading or spatial correlation, it cannot effectively combat interference. Therefore, neither the beamforming MIMO technique nor the pre-coding MIMO technique can achieve the best overall performance in application environments where the interference level may diverge greatly.